Audio amplifier system



Aug. 28, 1951 GQ F. DEVINE 2,556,057

AUDIO AMPLIFIER SYSTEM Filed on. 2. 1947 Fig. I.

Ihventor: Gecn'jge F. Devine,

- w aw His Attorney.

' Patented Aug. 28, 1951 AUDIO AMPLIFIER SYSTEM George F. Devine,Maccellus, N. Y., assignor to General Electric Company, a corporation ofNew York Application October 2, 1947, Serial No. 777,460

1 Claim. 1 My invention relates to audio amplifier systems, and moreparticularly to degenerative feedback circuits which may be employed insuch systems. It is a primary object of my invention to provide animproved degenerative feedback circuit for audio amplifier systems suchas those employed in phonograph reproducers.

In order to provide stability of operation and high fidelity soundoutput from an audio amplifier, it is customary to feed back a portionof the output voltage in degenerative phase to the input circuit. In thepast, such feedback voltage has been supplied in parallel with the inputvoltage. It is a particular object of my invention to provide a circuitfor applying a portion of the output voltage of an audio amplifyingsystem degeneratively in series with the input voltage, therebyobtaining improved fidelity of voltage output.

Another object of my invention is to provide means for applying aconstant percentage, independent of the volume control setting, of theoutput voltage of an audio amplifier degeneratively to the inputcircuit.

A further object of my invention is to provide, for a phonographreproducer, feedback means which permit a limited variation of the magnitude of the negative feedback voltage with the volume control setting,in order that the gain of the amplifier and its associated phonographpickup at maximum volume control setting may be established at adesirable level and a high degree of feedback may be utilized at lowvolume levels.

Another object of my invention is to provide a negative feedback circuitfor use in an audio amplifier system wherein the phase of the feedbackvoltage is invariant with the volume control setting.

A still further object of my invention is to provide means for obtainingsound output of fidelity comparable to that obtained by the use of basscompensation networks without using such networks, thereby effecting aconsiderable economy of parts and reduction of cost.

The features of my invention which I believe to be novel are set forthwith particularity in the appended claim. My invention itself, however,together with further objects and advantages thereof, may best beunderstood by reference to the following description taken in connectionwith the accompanying drawing, in which Fig. l is a schematic circuitdiagram of a audio amplifier which suitably embodies my invention, andFig. 2 is a modification of the circuit of Fig. 1, In the drawings, likereference numerals indicate like elements.

Referring to Fig. 1, an audio frequency amplifier, comprising anelectron discharge device I having a cathode 2, a control grid 3 and ananode l, is shown supplying audio frequency currents 2 to a loudspeaker5 through the conventional output transformer 6. I have shown a singlestage amplifier, however,

it will be understood that two or morestages may be used. The customarycathode bias resistor I and bypass condenser 8 are provided and positiveunidirectional energizing voltage, conventionally designated as +B, issupplied to the anode 4 of device I through the primary winding 9 ofoutput transformer 6.

A source of audio frequency signal voltage, which is shown comprising acrystal pickup III of a phonograph reproducer, is connected across avolume control potentiometer I I, the movable contact I2 of which isconnected to the control grid 3 of device I. as constituting a voltagesource in series with a capacitance. If the crystal is returned directlyto ground, as is the case in circuits using parallel feedback, it actsas a capacitive shunt across the feedback resistor, thus effectingundesirable phase shift in the feedback voltage. My circuit precludessuch phase shift, since the crystal III is returned in series with thefeedback resistor I6. Resistance I3 is shown shunting the output portionof potentiometer II. The function of resistance I3 is to offer a highimpedance load to the crystal pickup I0 at low level audio gain and alow impedance load for high level audio gain. Due to the capacitivenature of the crystal pickup I0, this variable loading providescontinuous bass compensation without the need for capacitors or othercircuit components and without appreciable loss of input voltage.

I provide means for applying a portion of the output voltagedegeneratively in series with the input circuit, such means as shownembodying a resistor I I connected between one side of the secondary I5of output transformer 6 and the low potential end of volume controlpotentiometer II, and another resistor I6 connected between the lowpotential end of potentiometer II and ground. The other side of thesecondary winding I5 of output transformer 6 is directly connected toground. In this manner, a portion of the output voltage is developedacross the feedback resistor IE and is supplied in series with thesignal input voltage appearing across resistance I3 to the grid 3 ofamplifier tube I. Since a 180- degree phase shift occurs in device I,the feedback voltage is in degenerative phase with the input voltage.Furthermore, the percentage of the output voltage so applied to theinput circuit is constant, depending only on the relative values of thefeedback resistances I4, I6, and is independent of the position of themovable contact I2 of volume control potentiometer II. By a properselection of the values of feedback resistances I4, I6, undesirableoverloading of the amplifier at maximum volume control setting is Forpurposes of illustration,

Crystal l0 may be considered the crystal pickup l and its loadingresistor 13 p are returned to the junction of the feedback resisors I4,I6. In this case the feedback voltage divider again comprises resistorsI4, l6 and the degenerative feedback voltage developed across resistanceI6 is again applied in series with the input voltage to the grid -3ofdevice I. In addition, acurrent limiting. resistance I1 is provided.

between. the high potential sideof. the grid leak resistance l3 and,the. grid 3 of device I. Further explanation of the circuit of Fig. 2 isomitted since... in allother respects the circuits of Figs. 1 and2' areidentical.

It will be.- seen that due to the resistive nature of. the. feedbackvoltage divider employed in the circuits of Figs. 1 and 2', the phase ofthe feedback voltage is independent of volume control adjustment.Furthermore, the percentage of the output. voltage which is fed back indegenerative phase. with the input voltage is independent of volumecontrol adjustment. As a further modific'ati'on, a limited variation ofthe magnitude and phase of the feedback voltage" with volume control.adjustment may be obtained" by employing suitable reactive elements inconjunction with the feedback voltage divider.

While I- have. shown and describedparticular embodiments of. myinvention, it will be understood. that numerous variations andmodificat'i'ons maybe made,.and I contemplate in the appended clai'mxtocover' all such modifications as fall" withinthe' true spirit andscope"of my invention;

What I claim as new and desire to secure by Letters; Patent of theUnited States is:

In" a phonograph reproducer of the type having a crystal pickup deviceadapted to generate alternating voltages extending over an audiofrequency band, said. device having, inherent capacitive re'actance; a,pair' of resistances connected in series circuit with said pickupdevice, said series circuit having a low potential point and a highpotential point, one of said resistances' being connected to said highpotential pointand having a variable tap, a third resistanceconnectcdbetween said tap and said low potential point, an electron dischargedevice having an input electrode connected to said tap, said devicehaving an output circuit having a grounded connection, and a connectionfrom a point on said output circuit displaced from said groundedconnection tothe commonterminal. of said pair of resistances, said-lowpotential point. being connected'to ground through a fourth resistance,the other of said pair of resistancesk and said fourth resistanceforming a feedback circuit. to supply a constant percentage of thevoltage of said output circuit in degenerativephase with respect to thecorresponding. input voltages within said band which are suppliedto saidinput electrode by said pickup device.

GEORGE: F. DEV-INE.

REFERENCES CITED The following referencesare' of record in: the fileofthis patent:

UNITED STATES PATENTS Great Britain June 1, 1939

